In surface ionization techniques, an analyte sample is first deposited on a plate surface, then desorbed from the surface and ionized by irradiation. The irradiation of the surface may be performed using a laser, or with a particle or ion beam, for example. The desorbed ions can be captured for analysis by a conduit leading to a mass spectrometer instrument using pneumatic and/or electrostatic forces. Currently, the most widely used surface ionization technique is matrix-assisted laser desorption ionization (MALDI). In this technique, samples are diluted in an ultraviolet(UV)-absorbing matrix material, then deposited on a sample plate, on which the mixed sample and matrix are co-crystallized by drying. A focused pulse of UV laser radiation is then directed onto the sample. The energy of the pulse is absorbed by the matrix material, which is desorbed from the surface and ionized, carrying with it analytes, which are ionized in turn by charge transfer processes.
In particle bombardment, a similar apparatus setup is employed, but instead of directing laser radiation onto the sample, a stream of particles, typically ions, is generated by electrospray ionization or another ionization process and is directed onto the sample. Analyte ions are generated by the impact of the ions on the sample, and related charge transfer processes that occur due to such impact.
In conventional configurations, the analyte ions generated using a MALDI or particle ionization source are drawn into the inlet of a straight conduit leading to a mass spectrometer. The proximal end of the conduit including the inlet is oriented orthogonally with respect to the sample-bearing plate surface. Typically, the conduit is also oriented along the longitudinal axis of the mass spectrometer, which is usually horizontal. This dictates that the sample-bearing surface be oriented vertically to be orthogonal to the conduit. This vertical configuration can be disadvantageous for certain applications. Moreover, the conduit is often not in the optimal position or orientation for receiving ions desorbed from the plate surface, reducing the collection efficiency of the ionization source.